Blender 2.91 Linear Animation acting like a Bezier
The problem is that you're using Quaternion (WXYZ) Rotation, which is the default setting for bones in Pose Mode. Set the Rotation method to XYZ Euler and animate the Y Rotation from 0° to 360°.
You should also set the last keyframe with 360° on frame 101 instead of 100 if you want a looped animation from frames 1-100, because 0° = 360° and the animation would look as if it pauses for one frame.